This invention relates generally to electrostatic spray coating apparatus and, more particularly, to an applicator or spray gun for use in such apparatus for atomizing and charging the coating material and further wherein the voltage charging the coating material may be selectively varied by the operator.
As suggested by U.S. Pat. Nos. 3,610,528, 3,731,145, 4,219,865, 4,377,838 and 4,491,276 the advantages and benefits of using electrostatic spray coating have long been recognized. The noted prior art discloses an evolutionary process in applicator design extending from spray guns which relied upon external power sources to produce the charged atomized coating material to spray guns having self-contained systems for producing voltage for charging the coating material being dispensed by the gun. In particular, two U.S. Pat. Nos., 4,219,865 and 4,290,091 (to Malcolm; owned by the assignee of the present invention) disclose the type of spray gun which has an entirely self-contained electrical power supply for charging the dispensed coating material. This type of spray gun provides the advantage of not requiring connection to an external power supply, thereby eliminating the need for power cords extending from the gun to an external power supply which adds weight to and reduces the mobility of the gun.
Another improvement manifested in the prior art is disclosed in U.S. Pat. Nos. 4,266,262, 4,674,003 and 4,745,520. These patents are directed generally to the concept of controlling or sensing the current which is provided to charge the coating material. U.S. Pat. No. 4,745,520 provides for sensing the output current whereby the sensed current controls the supply of operating potential. Similarly, U.S. Pat. No. 4,266,262 provides for control by a sensing signal which is transmitted to a counter circuit controlling a switching circuit which connects only a portion of alternating current cycles to the input depending upon the magnitude of the sensed signal. U.S. Pat. No. 4,674,003 includes a microcomputer for identifying the current flowing between the charging electrode and the workpiece whereby the optimum spray effect or constant high voltage is maintained.
Despite the improvements and advantages offered by the above-noted patents and refinements in electrostatic spray coating technology, there are some problems which have remained unaddressed. Some of these problems are addressed in a copending application (U.S. Ser. No. 446,810 now abandoned) owned by the assignee of the present invention, entitled Solvent Resistant Electrostatic Spray Gun.
The present invention is directed to another one of the problems which has remained unsolved in the prior art. Specifically, different paints or coating materials may require different charging voltages so that an appropriate and optimal finish may be achieved. Also the distance between the spray gun and the workpiece may vary and the shape of the workpiece may vary. Therefore, it would be advantageous if the operator of the spray gun could vary, conveniently and rapidly, the spraying voltage without resorting to electromechanical switches or wire mechanical linkages on the body of the spray gun and without resorting to a remote adjustment location for adjusting the voltage being supplied to the gun, thereby adjusting the spraying voltage with which the coating material is being charged.